1. Field of the Invention
The present invention relates to a method and device for protecting an image displaying device, and more particularly, to a method and device for protecting a cathode ray tube in an image displaying device by using a brightness control circuit.
2. Description of the Prior Art
Many TV sets and computer displays use a cathode ray tube as the main displaying element. The cathode ray tube of the prior art comprises a screen, an electron gun, and an anode. The electron gun comprises a cathode capable of emitting electrons, and a cylinder electrode capable of concentrating the electrons together and then accelerating them to be focused on the screen. The electron flow from the electron gun is controlled by image control signals and is projected onto a fluorescent screen to create images. The electron flow emitted by the electron gun gathers at the anode of the cathode ray tube to form an anode current. The magnitude of the anode current reflects the magnitude of electron flow emitted by the electron gun. The fluorescent screen of the cathode ray tube is coated with fluorescent material, and thus will illuminate when hit by the electrons emitted by the electron gun. If the image is too bright, the quantity of electrons hitting the fluorescent screen within a short period of time will be high. This may lead to early degradation of the fluorescent material coating and a reduced cathode ray tube lifespan. Displaying devices using cathode ray tubes are usually equipped with an automatic brightness limiting circuit (ABL circuit) for this purpose. When the average brightness exceeds a predetermined value, the ABL circuit will automatically be activated to prevent burning out of the fluorescent material as well as protecting other circuits from harm.
It is therefore a primary objective of the present invention to provide a method and device for protecting a cathode ray tube in an image displaying device by using a brightness control circuit as a new ABL circuit to solve the above mentioned problem.
In a preferred embodiment, the present invention provides a displaying device and a method for adjusting the brightness of a displaying device, the displaying device comprising:
an image signal amplifier for amplifying incoming image signals;
a cathode ray tube comprising a screen at its front end, an electron gun at its rear end for projecting and controlling an electron beam according to the amplified image signals onto the screen to create an image picture, and an anode for collecting the electron beam emitted onto the screen to produce an anode current; and
a brightness control circuit electrically connected to a control end of the image signal amplifier for controlling the amplification magnitude of the image signal amplifier and electrically connected with the anode of the cathode ray tube via a flyback transformer which is used for providing the anode with the anode current wherein when the incoming image signals make the image picture shown on the screen too bright, the anode current of the anode will be increased accordingly, and the brightness control circuit will reduce the amplification magnitude of the image signal amplifier at the same time according to the increase of the anode current so as to reduce the anode current, and wherein the brightness control circuit comprises an adjusting circuit electrically connected with the control end of the image signal amplifier for adjusting the brightness of the image picture of the screen;
the method comprising:
using a brightness detector to detect the brightness of the image picture of the screen, and
using the adjusting circuit to adjust the brightness of the image picture until the brightness detected by the brightness detector reaches a predetermined brightness value.
It is an advantage of the present invention that a brightness control circuit adjusts the relationship between the anode current and the amplification magnitude of the image signals. When the image signals from the computer are such that the brightness of the image screen is too great, the anode current of the cathode ray tube increases. The brightness control circuit 16 then detects this change and sends a signal to reduce the amplification magnitude of the image signal amplifier. This protects the screen 18 from an electron beam that is too strong, thus maintains its useful life as well as protecting other circuits from damage caused by excessive average anode current.
This and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiment which is illustrated in the various figures and drawings.